Voltage-controlled ionic variable resistor employing material transfer

ABSTRACT

An ionic variable resistor comprises a base metallic film, an intermediate film of a solid ionic conductor containing a cation similar to the metal comprising the base layer, and an upper film layer of the same metallic material as the base layer. A source of potential connected across the device causes transference of material from the upper layer to the lower layer, or vice versa, depending on the polarity, to effect a change in resistance in the metal films. The variable resistor may be advantageously applied by vacuum deposition to microelectronic wafers, thin film substrates, or circuit boards.

United States Patent [72] Inventor Charles Feldman 7400 Rebecca Drive,Alexandria, Va. 22307 [21] Appl. No. 692,738 [22] Filed Dec. 22, 1967(45] Patented Sept. 28, 1971 [54] VOLTAGE-CONTROLLED IONIC VARIABLERESISTOR EMPLOYING MATERIAL TRANSFER 16 Claims, 5 Drawing Figs.

[52] U.S.Cl 317/231, 317/237, 317/235 [51] lnt.Cl ll0lg9/14 [50] Fieldof Search 338/20, 32,

[56] References Cited UNITED STATES PATENTS 2,075,733 3/1937 Lazarus338/20 2,751,477 6/1956 Fitzgerald 338/20 2,773,250 12/1956 Aigrain etal.. 317/235 3,307,089 2/1967 Yamashita 317/235 3,355,637 11/1967Johnson 317/235 3,436,668 4/1969 Russell 317/235 830,924 9/1906Pawlowski 317/237 1,891,097 12/1932 Krauss 317/237 1,930,519 10/1933lrion 317/237 1,900,018 3/1933 Lilienfeld 317/231 Primary Examiner.lamesD. Kallam Attorney-Larson, Taylor and Hinds ABSTRACT: An ionic variableresistor comprises a base metallic film, an intermediate film of a solidionic conductor containing a cation similar to the metal comprising thebase layer, and an upper film layer of the same metallic material as thebase layer. A source of potential connected across the device causestransference of material from the upper layer to the lower layer, orvice versa, depending on the polarity, to effect a change in resistancein the metal films. The variable resistor may be advantageously appliedby vacuum deposition to microelectronic wafers, thin film substrates, orcircuit boards.

SOURCE PATENTEnsiPzslsn $609,469

8 SOURCE 4Q ELECTRICAL 3O CIRCUIT F/G. 4 I l A I R5 SOURCE-I ELECTRICALJ CIRCUIT INVENTOR CHARLES FELDMAN BY Jug Ja W ATTORNEY 5VOLTAGE-CONTROLLED IONIC VARIABLE RESISTOR EMPLOYING MATERIAL TRANSFERFIELD OF THE INVENTION The present invention relates to variableresistance devices and more particularly to variable resistance devicesusable in microelectronic circuits.

BACKGROUND OF THE INVENTION AND THE PRIOR ART The present trend towardminiaturization of electronic components has resulted in heavy demandsfor components corresponding to standard-sized components which may beadapted for use in the reduced-size circuits. Furthermore, there havebeen demands for new types of microminiature components capable of beingadjusted or altered by control signals transmitted from a distance forspace applications and the like. One particularly difficult problem hasbeen that of providing a satisfactory variable resistance element andparticularly of providing a resistance element that may be reversiblyvaried, i.e., a resistance element whose resistance may be selectivelyincreased or decreased. A second problem is that of providing asatisfactory trimming or tuning resistor used in increasing theprecision of microcircuits, e.g., in achieving a precision frequencyfilter.

ln large-sized systems, a number of resistance varying techniques areavailable, e.g., hand adjustable rheostats, and servomotors connected topotentiometers, but in microminiature systems which are too small forknobs, techniques available for variation of resistance are very few innumber and in general rather crude. Typical microcircuit techniquesinclude scratching or abrading of an exposed film resistor, by use of asuitable tool. It will, of course, be appreciated that the originalresistance value of the resistor cannot be restored once part of theresistance material has been scraped off and thus this techniqueprovides variation of resistance in one sense only.

Although voltage variable resistance elements per se are known, suchelements are not in general suitable for microminiature use because oftheir sizes and shapes. One such known device employs a liquidelectrolyte which must be encapsulated thus generally limiting the useof the device to larger sized circuits. Other presently known so-calledminiature variable or reversible resistors require the use of miniaturesockets or screws.

BRIEF SUMMARY OF THE INVENTION The present invention provides a variableresistance device which is particularly adapted for use inmicroelectronic circuitry. The resistance device of the invention is ofcompletely solid state construction, generally comprising layers of thinfilms, and thus the space requirements are extremely low.

In accordance with a presently preferred embodiment of the inventionthere is provided a base resistance layer comprising a thin metallicfilm mounted on a nonconducting substrate, an intermediate layercomprising an ionic conductor containing a metal ion or cation similarto the metal of the resistance layer and an upper layer comprising ametallic film of the same material as the resistance layer. Inoperation, the resistance of the device is altered by applying apotential thcreacross such that material from the upper layer istransferred by ionic conduction through the intermediate conductor tothe base layer. As the material builds up on the base layer theresistance thereof decreases. Similarly with a potential of oppositepolarity applied across the device, material will be removed from thebase layer and resistance of the base layer will be increased. It shouldbe noted that it is also possible to utilize an ionic conductor having amobile anion rather than a mobile cation.

In accordance with one feature of the invention both the base layer andthe upper layer may be used as circuit elements, the resistance of onelayer increasing while the other decreases and vice versa (as in apotentiometer).

Other features and advantages of the present invention will becomeapparent upon consideration of the following description of the drawingswherein:

FIG. 1 is a schematic representation of a resistance device inaccordance with the present invention,

FIG. 2 is a perspective view of an embodiment of the invention whereinthe resistance device of FIG. I located in a microminiature circuitboard arrangement,

FIG. 3 is a perspective view of a thin film circuit embodiment of theinvention wherein the resistance device acts as a trimmer for a furtherresistance,

FIG. 4 is a schematic circuit diagram of a further embodiment of theinvention, and

FIG. 5 is a schematic representation of yet another embodiment of theinvention.

Referring to FIG. 1 of the drawings, there is shown a solid statevariable resistance device generally denoted RI which comprises a baselayer I mounted on a nonconductive substrate 2, an intermediate layer 3and an upper layer 4. It will be understood that FIG. 1 is merelyillustrative of the construction of resistance device R1 and that thethicknesses and proportions shown therein are not to be taken asdefinitive.

Base layer 1 is constructed of a metallic resistance material and ifformed on substrate 2 as a very thin film by a suitable method such asvacuum deposition. The metallic material used may, for example, be ametal such as silver or copper. It is noted that base layer 1 may itselfbe made up of a plurality of layers of material. For example, base layer1 may comprise a base layer of relatively stable resistive metal such asplatinum or chromium and an upper layer of a relatively unstableresistive metal such as silver. In this way a more stable overallresistance device is produced and the value of the resistance may bevaried only within a relatively narrow range for purposes of trimming.Alloys or mixtures of metals may also serve as the base layer. As willbecome clear from the description of the operation of the device as setforth hereinbelow it is essential that the layer of base layer 1 whichis in contact with intermediate layer 3 be of a metal that gives up itsmetal ions relatively freely.

Base resistance 1 may be made extremely thin and may even be madelacunary." The term lacunary structure as used herein means a layer sothin that it is not continuous, the layer constituting, in effect,islands of conductive material. An electrical resistor including such aconductive layer is disclosed in my earlier U.S. Pat. No. 2,984,589.Such layers are extremely sensitive to thickness changes.

Intermediate layer 3 comprises an ionic conductor which contains a metalion or cation similar to the metal comprising the base layer 1. Theionic conductor 3 can, for example, be a silver or copper halide orsulfide or a mixture thereof. Possible combinations include a mixture ofsilver iodide with sliver chloride. Other possibilities include amixture of copper bromide and copper iodide and mixtures of varioushalides of thallium although in general any stable ionic conductorwherein the metal ion is capable of movement may be used. It is notedthat the additive of an impurity such as Tellurium to a material such asAgCl also improves the cation mobility. Intermediate layer 3 may bevacuum deposited on base layer 1.

Upper layer 4 may be constructed of the same material as base layer 1and may be vacuum deposited on intermediate layer 3. An importantfeature of the present invention is that upper layer 4 need not be andpreferably is not deposited over the entire surface of layer 3. Toprovide trimming" in a manner described hereinbelow layer 4 need onlycover a small portion of the base resistor 1 and thus the capacitance ofthe device may be kept at a minimum.

Resistance device RI further includes a pair of output leads 5 and 6connected to base resistance 1 as shown and a control lead 7 connectedto upper layer 4. Lead 7 connects layer 4 to a variable source ofpotential denoted 8. Source 8 may, for example, simply comprise a switchand a DC battery although it would be appreciated that source 8 can takea number of other forms.

In the operation of the device of FIG. 1, resistance layer 1 and 4 serveas electrodes or plates separated by the solid state electrolyte formedby ionic conductor 3. By applying a potential supplied by source 8across the layers 1 and 4, a portion of the metallic resistance materialfrom layer 4 may be transferred through ionic conductor 3 to baselayer 1. As this material builds up on base layer 1 the resistance oflayer 1 decreases. In a like manner, with the potential supplied bysource 8 of an opposite polarity, metallic material from layer 1 may betransferred to layer 4. Under these latter circumstances the resistanceof layer 1 is increased.

The amount of solid material or metal built up on a layer is governed bywell-known laws of electrochemistry. The material transferred, M, isproportional to the total charge, Q, passed according to the relation:

M=Qml VF where F Faradays constant, Q =charge transfer, and m atomicmass of the ion being transferred,

V= valence of the mobile ions.

The charge transferred Q=it=( V/R)t. where 1' current passed, t is timeelapsed, and

v voltage and R the resistance of the ionic conducting layer.

In a given layer the mass built up may be controlled by either thevoltage or the time, depending on the particular system and the voltageand time available.

For variable resistors, R should be made large, i.e., many times largerper unit area than the under or over lying resistance layerscorresponding to layers 1 and 4 of FIG. 1. In addition R should, ofcourse, be maintained solely an ionic conductor and no electronicconduction should take place.

Because, for most applications, only the relative amounts of materialcomprising layer 1 are important, the material from layer 4 need not beplated uniformly thereon and there need be no concern with edge effects.This means, as set forth hereinabove, that the configuration of upperlayer 4 need not bear any particular relationship to that of base layer1 and need not cover the entire surface thereof.

FIG. 2 is a largely schematic representation of the resistance device ofthe present invention embodied in a circuit board assembly. FIG. 2 isnot drawn to scale and the relative sizes of the elements shown thereinshould not be taken to be an accurate depiction of the actual relativesizes. Resistance device R2 corresponds to resistance device R1 of FIG.1 and corresponding elements thereof have been given the same numberswith primes attached. The assembly as shown is comprised of a pair ofcircuit boards BI and B2. A plurality of components generally denoted CIand C2 are mounted on boards BI and B2, respectively, in a conventionalmanner. Resistance device R2 is mounted on lower board B1 and inaccordance with an important feature of the invention may be depositedthereon along the other components Cl. It will be further appreciatedthat because the ionic resistance device may be made very thin it isparticularly suitable for use in circuit arrangements utilizing closelystacked mounting boards such as that shown. Variation of the resistancevalue of the device may be effected by varying the voltage on upperlayer 4' as set forth hereinbefore. An input to layer 4' is provided byextending lead 7' through circuit board B2 so that a variable source ofpotential (not shown) may be applied. The operation of resistance deviceR2 is the same as described in connection with resistance device R1 ofFIG. 1.

FIG. 3 shows an embodiment illustrating the use of the resistance deviceof the invention in a vacuum deposited thin film circuit. This circuitcan be made very small and the ionic resistor of the present inventionparticularly suited for use therein. The circuit may include a number ofvarious components including a fixed resistor Rf, a capacitor C, atransistor T, and an ionic element R4 according to the invention. Allelements may be formed by the same vacuum technique-on a glass orceramic substrate S. The element of the invention is thus compatiblewith various curved forms of microelectronic circuits. In thisembodiment ionic resistor R4 is connected in parallel with and utilizedas a trimming resistor for a main fixed resistor Rf so that the value ofresistor Rf may be varied within predetermined narrow limits.

The utilization of the device of the present invention in a completedcircuit provides substantial advantages as compared with theconventional scratching or abrading techniques described above. First,the device of the invention provides variation of resistance in bothsenses, i.e., the resistance of the device may be increased as well asdecreased. Second, the change in resistance of the device may beaccomplished simply by varying the voltage on the upper layer or plate(element 4 or 4) through an external contact or electrode. This permitsfinal adjustment ofa circuit after packaging or sealing.

Further, in this regard, a plurality of resistance devices may beutilized and under specialized circumstances may be controlled by asingle source. A second resistance device R3 is shown in dotted lines inFIG. 2 as being mounted on board B1 and also as having an input lead 7'extending through board B2. Depending, of course, on the nature of thecircuit in which they are included, devices R2 and R3 may besimultaneously controlled by a common source of potential. For example.the resistances R2 and R3 might be corresponding elements in like stagesof a multistage circuit.

FIG. 4 shows a further embodiment of the invention wherein bothresistive layers of the resistance device are utilized as circuitelements. Resistance device R5 corresponds to device R1 of FIG. 1 andcorresponding elements thereof have been given the same numbers withdouble primes attached. Resistance layers 1" and 4" are included incircuits denoted A and B respectively. Because a variation in thepotential supplied by source 8" causes a variation in the resistancevalue of layer 4" as well as layer 1" both layers may be utilized ascircuit elements. As set forth above in regard to FIG. I application ofa potential of one polarity causes transference of material from theupper layer 4" will increase for this potential while the resistance ofbase layer 1" decreases. This effect of simultaneously oppositelyvarying resistances may, for example, be utilized in balancing circuits,and the like.

In accordance with a further variation of the device one or bothresistor layers corresponding to layers 1 and 4 of FIG. 1 may bereplaced by a semiconductor layer. The addition of even a small amountof impurity ions to the surface of a semiconductor changes theresistance thereof greatly. The general operation of the unit would bethe same as set forth hereinbefore. The semiconductor may conventionallybe Si or Ge although CdS-ZnS mixtures or even metal oxides may also beused.

In accordance with a further feature of the present invention thevariable resistance device may be heated slightly to increase the ionicconduction of the intermediate layer during trimming. The temperature ofthe device may be elevated to the l00-200 C. range which in general willnot damage the elements of the circuit. This heating will increase thespeed of the resistance change. For example, heating Agl above 146 C.decreases the resistance thereof by a factor of about 1,000 due to acrystal transformation.

The stability of the unit after formation, with respect to both time andtemperature, is an important factor. The question of whether the ionswill diffuse or drift into a desirable position must be attended to. Toobtain good stability, at number of steps could be taken as depending onthe particular requirements of the system. First, material with very lowionic conductivity can be used so that only upon application of a largevoltage would one expect to obtain migration. Second, a barrierconsisting of a different material can be formed so that an additionalpotential above that normally used would be necessary to cause an iontransfer. Third, the ionic conductor can be made thick so that at thevoltages usually used (millivolts) no drift would take place. Fourth,ionic conductors may be used which as described previously requirethermal activation to permit charge transfer. Fifth, the electrodes ofthe material,

in some cases, can be connected so that a minimum voltage appears acrossthe ionic conductor during actual use.

In accordance with a specific embodiment of the invention wherein layer1 is sliver, layer 3 is silver iodide and layer 4 is also silver thefollowing exemplary dimensions, etc., may be observed:

1. Thickness of Agl=lp=l O cm.

2. lonic resistivity of electrolyte lohm/cm.

3. Applied potential l v. (This gives a field of l0v./cm.,

well below the breakdown strength.)

4. Current density at l v. l0 amps/cm.

5. Mass transfer of Ag from Faraday Law under conditions (l), (3) and(4): m==l0 grams/sec. cm. or 100 AU/sec.

Thus a I00 AU silver film would change by 100 percent in one (l) secondwith l v. applied. This is more than would be required for a simpletrimming resistor. The resistance change in thin films will beconsiderably larger than the mass change since the film does not havebulk behavior but is much more sensitive to thickness variations. Theprecise change depends on the texture of the film (size and number ofgrains) and how the new silver deposits on the surface, i.e., whether itbuilds up on the grain; whether it migrates, etc.

It is noted that in the specific embodiment discussed hereinbefore ithas been assumed that the cation or metal ion of the intermediate layeris the mobile ion. It is noted that it is possible to achieve the sameeffect with only the anion mobile. In this latter situation, there ischemical conversion of the film surface rather than plating operation.As stated, the net effect, i.e., variation of the resistance of thesample resistor, would be the same. It is further possible to utilize asystem wherein both types of ions, i.e., the cations and ions, aremobile. Examples of some conductors in which only the cation is mobileare silver chloride and silver bromide. Calcium fluoride and leadchloride are examples of conductors in which only the anion is mobile.

It will be appreciated by those skilled in the art that variations ofthe basic physical construction of the device are possible. For example,a second main resistance layer corresponding to base layer 1 may beutilized. This second layer could be deposited on a second ionicconductor layer deposited on the upper surface of upper layer 4, theresultant device being made up of five layers in all. With thisconstruction variation of the voltage on the central layer(corresponding to layer 4) will cause variation of the resistances ofboth the base and uppermost resistive layers. Such a construction isshown in FIG. 5, wherein the base and uppermost resistive layers aredenoted la and lb, first and second ionic layers are denoted 3a and 3band the layer corresponding to layer 4 is denoted 4a. It is noted thatthe intermediate ionic layers may themselves be made up of layers havingdifferent conductivities. Thus, referring to H6. 5, ionic layer'3a ismade up of layers 3a and 3a" of different conductivities.

It will be further understood that the variable resistance device of thepresent invention has many uses, such as in the tuning of circuits, theadjusting of the characteristics of filters, the variation of variouscircuit voltages and in self adaptive circuits in response to externalor internal signals.

Having thus described the invention in the manner required by the PatentStatutes, I wish it to be understood that the foregoing disclosure isillustrative rather than definitive, and that the scope of the inventionis defined by the subjoined claims interpreted in the light of thespecification and drawing.

lclaim:

l. A variable resistance device comprising a solid state elementcomprising, an intermediate layer located between first and second outerlayers of a metallic material and comprising a solid ionic conductorincluding a mobile ion, and voltage means for supplying a potential toone said outer layer to cause a transfer of material to one of saidouter layers to produce a change in the resistance of the device whichpersists in the absence ofsaid potential.

2. A variable resistance device in accordance with claim 1 wherein saidionic conductor contains a metal ion similar to the metal comprising oneof said outer layers.

3. A variable resistance device in accordance with claim I wherein saidlayers comprise vacuum-deposited films.

4. A variable resistance device in accordance with claim 3 wherein atleast one of said layers is a lacunary structure.

5. A variable resistance device in accordance with claim 1 wherein saidouter layers comprise a base layer and an upper layer, wherein saidpotential is supplied to said upper layer, wherein the metallic materialof the base layer and the upper layer is silver, and wherein saidintermediate layer includes a halide of silver.

6. A variable resistance device in accordance with claim 1 wherein onesaid outer layers forms a resistive element in an electrical circuit.

7. A variable resistance device in accordance with claim 6 wherein theother of outer said layer also forms a resistance element in anelectrical circuit.

8. A variable resistance device in accordance with claim 1 wherein oneof said outer layers includes a relatively stable metal and a metalwhich gives up its metal ions relatively freely.

9. A variable resistance device in accordance with claim 1 wherein bothouter layers are the same material.

10. A variable resistance device in accordance with claim 1 wherein saidintermediate layer comprises a further solid ionic conductor ofdifferent conductivity from the first-named solid ionic conductor.

11. A variable resistance device comprising a solid state elementcomprising a base layer of an electronically conductive material, andintermediate layer comprising a solid ionic conductor having a mobileion and an upper layer of an electronically conductive material, andvoltage means for supplying a potential to one of said upper and lowerlayers to cause a transfer material to one of the layers to therebyproduce a change in the resistance which will persist in the absence ofsaid potential, said electronically conductive material of said baselayer comprising a semiconductor material.

12. A microelectronic circuit comprising a substrate, a plurality ofcircuit elements deposited on said substrate, said circuit elementsincluding a solid state resistance device comprising a base layer of ametallic material, an intermediate layer of an ionic conductorcontaining a cation similar to the metal comprising said base layer, andan upper layer of metallic material, and voltage means for supplying apotential to said upper layer to cause a transfer of material throughsaid intermediate layer to one of said other layers to produce a changein the resistance of the device which persists in the absence of saidpotential.

13. A microelectronic circuit in accordance with claim 12 wherein saidsubstrate comprises a first circuit board and wherein the circuitincludes a second circuit board lying above said first circuit board anda connection extending through the uppermost of said circuit boards forconnecting said upper layer of said solid state resistor to said voltagemeans.

14. A microelectronic circuit in accordance with claim 12 wherein saidlayers are vacuum-deposited films.

15. A microelectronic circuit in accordance with claim 12 wherein saidplurality of circuit elements comprise a plurality of solid state ionicresistance devices, said voltage means constituting means forcontrolling the resistance of said plurality of solid state resistancedevices.

16. A variable resistance device comprising a solid state elementcomprising a base layer of an electronically conductive material, andintermediate layer comprising a solid ionic conductor having a mobileion and an upper layer of an electronically conductive material, andvoltage means for supplying a potential to said upper layer, saidelectronically conductive material of said base layer comprising asemiconductor material.

2. A variable resistance device in accordance with claim 1 wherein saidionic conductor contains a metal ion similar to the metal comprising oneof said outer layers.
 3. A variable resistance device in accordance withclaim 1 wherein said layers comprise vacuum-deposited films.
 4. Avariable resistance device in accordance with claim 3 wherein at leastone of said layers is a lacunary structure.
 5. A variable resistancedevice in accordance with claim 1 wherein said outer layers comprise abase layer and an upper layer, wherein said potential is supplied tosaid upper layer, wherein the metallic material of the base layer andthe upper layer is silver, and wherein said intermediate layer includesa halide of silver.
 6. A variable resistance device in accordance withclaim 1 wherein one said outer layers forms a resistive element in anelectrical circuit.
 7. A variable resistance device in accordance withclaim 6 wherein the other of said outer layer also forms a resistanceelement in an electrical circuit.
 8. A variable resistance device inaccordance with claim 1 wherein one of said outer layers includes arelatively stable metal and a metal which gives up its metal ionsrelatively freely.
 9. A variable resistance device in accordance withclaim 1 wherein both outer layers are the same material.
 10. A variableresistance device in accordance with claim 1 wherein said intermedIatelayer comprises a further solid ionic conductor of differentconductivity from the first-named solid ionic conductor.
 11. A variableresistance device comprising a solid state element comprising a baselayer of an electronically conductive material, and intermediate layercomprising a solid ionic conductor having a mobile ion and an upperlayer of an electronically conductive material, and voltage means forsupplying a potential to one of said upper and lower layers to cause atransfer material to one of the layers to thereby produce a change inthe resistance which will persist in the absence of said potential, saidelectronically conductive material of said base layer comprising asemiconductor material.
 12. A microelectronic circuit comprising asubstrate, a plurality of circuit elements deposited on said substrate,said circuit elements including a solid state resistance devicecomprising a base layer of a metallic material, an intermediate layer ofan ionic conductor containing a cation similar to the metal comprisingsaid base layer, and an upper layer of metallic material, and voltagemeans for supplying a potential to said upper layer to cause a transferof material through said intermediate layer to one of said other layersto produce a change in the resistance of the device which persists inthe absence of said potential.
 13. A microelectronic circuit inaccordance with claim 12 wherein said substrate comprises a firstcircuit board and wherein the circuit includes a second circuit boardlying above said first circuit board and a connection extending throughthe uppermost of said circuit boards for connecting said upper layer ofsaid solid state resistor to said voltage means.
 14. A microelectroniccircuit in accordance with claim 12 wherein said layers arevacuum-deposited films.
 15. A microelectronic circuit in accordance withclaim 12 wherein said plurality of circuit elements comprise a pluralityof solid state ionic resistance devices, said voltage means constitutingmeans for controlling the resistance of said plurality of solid stateresistance devices.
 16. A variable resistance device comprising a solidstate element comprising a base layer of an electronically conductivematerial, an intermediate layer comprising a solid ionic conductorhaving a mobile ion and an upper layer of an electronically conductivematerial, and voltage means for supplying a potential to said upperlayer, said electronically conductive material of said base layercomprising a semiconductor material.